This invention relates to boiling water nuclear reactors. Such nuclear reactors increase their power output by two well known expedients.
The first of these expedients is the increase in coolant flow through the reactor. Simply stated, increased coolant flow increases the amount of moderator present in the reactor. Fast neutrons from the nuclear reaction are moderated in greater number, promoting additional nuclear fission reactions and power output increases.
Alternately, the nuclear reaction can be controlled by so-called "control rods". These rods when inserted within a reactor core absorb thermal neutrons and inhibit the nuclear reaction. When control rods are withdrawn, less thermal neutrons are absorbed. Instead of being absorbed, the thermal neutrons find their way into the promotion of further nuclear fission reactions. Power output increases,
Those having skill in this art will realize that the above description constitutes a gross simplification; this simplification can help in the understanding of materials that follow.
Nuclear fuels are typically arranged in fuel bundles. The fuel bundles themselves contain side-by-side tubes, the tubes being filled, and sealed at both ends with the fissionable material trapped inside. The water coolant in the reactor is relied upon to both moderate the fast neutrons and extract heat from the individual fuel rods. In the absence of the extraction of the heat from the individual fuel tubes, damage to the fuel can occur.
One type of damage that can occur to tubes within a fuel bundle results from a departure from nucleate boiling. In nucleate boiling individual steam bubbles form on the tube surface (at so called bubble nucleation) as heat is transferred to the coolant. As the bubbles rapidly form and leave the tube surface a very agitated coolant condition exists at the tube surface promoting a very efficient heat transfer process--nucleate boiling.
When a departure from nucleate boiling occurs a steam film forms adjacent to the wall of the tube. The steam film is inefficient in extracting heat from the tube. When such a steam bubble forms, it is possible that the metal of the tube can become overheated from the nuclear reaction and the structural integrity of the tube can be lost.
To make absolutely certain that this type of casualty does not occur, all fuel bundles in boiling water reactor configurations are assigned bundle power limits to prevent a departure from nuclear boiling.
Other types of damage to a fuel rod can occur as a result of an overpower condition even while operating in the nucleate boiling regime. The power level of a fuel rod determines the temperature distribution within the rod. A higher power level requires a higher rod operating temperature to drive the nuclear generated heat out of the rod to the coolant. Operation of fuel rods at too high a power level can result in fuel melting or fuel expansion that strains the confining tube the extent that tube failure occurs. These types of rod failure mechanisms depend on the power generated per unit length of fuel rod tube.
A third type of catastrophic tube failure condition is possible during severe loss of coolant accident (LOCA). During a LOCA the moderator coolant is lost between the fuel rod tubes. The loss of the heat transfer medium causes the residual decay heat from the nuclear fuel to rapidly heat up the fuel tubes to high temperatures. At these high temperatures radiation heat transfer between tubes is a significant heat transfer process. It is a characteristic of radiation heat transfer to tend to transfer heat from hotter fuel tubes in the fuel bundle to colder tubes. Thus the peak fuel tube cladding temperature during a LOCA has been found to be limited by controlling the average fuel rod power at each axial elevation of a fuel bundle prior to a LOCA. This is possible because fuel rod residual decay heat power during a LOCA is directly proportional to operating fuel rod power prior to the LOCA.
If fuel tubes become too hot (in excess of approximately 2200.degree. F.), the zircaloy alloy tubing metal chemically reacts vigorously within the steam environment. The chemical reaction releases combustible hydrogen gas and embrittles fuel tube cladding. High temperature cladding has reduced integrity for containing fuel and radioactive materials and is subject to shattering from the thermal shock of rapid cool down when the reactor system is reflooded with water during LOCA recovery. Limits are therefore imposed on the maximum average fuel rod operating power in a fuel bundle at each axial elevation prior to a LOCA, to limit the peak fuel rod tube cladding temperature that could be reached during a LOCA.
The types of operating thermal limits can therefore easily be summarized.
First, since the overall power output of a fuel bundle can result in a departure from nuclear boiling the overall power output of each fuel bundle is monitored to maintain nucleate boiling. The bundle power at which departure from nucleate boiling is predicted to occur, the critical power, is divided by the monitored bundle power and the ratio parameter termed the bundle critical power ratio, CPR. The CPRs of all bundles must exceed unity to prevent a departure from nucleate boiling.
Second, it is of concern that no rod anywhere within a fuel bundle at any point exceed design temperatures. Since fuel rod temperatures are determined by the rod power per unit axial length, operating limits on rod linear power (power per unit length) are established. The operating linear powers of all sections of all fuel rods are effectively monitored and compared to the limits during operation.
Finally, within each fuel bundle the average linear power at each elevation is determined and compared to limits to assure acceptable consequences during a potential LOCA.
The classification of the above thermal limits is also subdivided. A first thermal limit is chosen and denominated as an "operating thermal limit". This operating thermal limit is a limitation of normal day to day steady operation. It is the object of routine nuclear plant operational power increases not to exceed these so-called operating thermal limits. Operating thermal limits include margin allowances for unplanned power increases or heat transfer degradation as might occur during abnormal system transients or accidents.
In addition to the operating thermal limits, there is a second and more stringent limits known as safety thermal limits. The safety thermal limits reside at or near the point where damage to the fuel tubes can occur. Obviously, the goal of plant operation is to remain within operating thermal limits so that safety limits are never violated. Plant instrumentation is provided to assure that operating and safety limits are not violated on operator initiated power increases by core flow increases and control rod withdrawal.